The sheet materials to which this invention is directed are usually referred to in the art as "electrical" silicon steels or, more properly, silicon-irons and are ordinarily composed principally of iron alloy with about 2.2 to 4.5 percent silicon and relatively minor amounts of various impurities and very small amounts of carbon. These products are of the "cube-on-edge" type, more than about 70 percent of their crystal structure being oriented in the (110) [001] texture, as described in Miller Indices terms.
Such grain-oriented silicon-iron sheet products are currently made commercially by the sequence of hot rolling, heat treating, cold rolling, heat treating, again cold rolling and then final heat treating to decarburize, desulfurize and recrystallize. Ingots are conventionally hot-worked into a strip or sheet-like configuration less than 0.150 inch in thickness, referred to as "hot-rolled band". The hot-rolled band is then cold rolled with appropriate intermediate annealing treatment to the finished sheet or strip thickness usually involving at least a 50 percent reduction in thickness, and given a final or texture-producing annealing treatment. As an alternative practice, as set forth in U.S. Pat. No. 3,957,546 (Fiedler), the hot-rolled band is cold rolled directly to final gauge thickness.
In these boron- and nitrogen-containing silicon-irons, strong restraint to normal grain growth and thus promotion of secondary recrystallization to a precise (110) [001] grain orientation is the result of controlling the ranges of these constituents. The sulfur effective for this purpose is that which is not combined with strong sulfide-forming elements such as manganese, a presently unavoidable impurity in iron and steel. Thus, the total sulfur is necessarily greater than that necessary to provide its grain growth inhibition effect.
It is also generally recognized in the art that the presence of high total sulfur and a small quantity of boron can lead to marked brittleness in welds made in the silicon-iron alloy. Due to weld brittleness, it has not been generally possible to weld two hot rolled coils together for cold rolling, as would be a desirable operating practice, since improvements in weldability gained by reducing the sulfur content would be offset by the resulting sacrifice of magnetic properties of the alloy.
Accordingly, there is a substantial need to the art for grain-oriented silicon-iron sheet having both improved magnetic properties and good weldability.